Step 1 Using pinch bar, get between rotten tread and supporting cleat beneath to remove. If your tread is housed into stringer, you will have to cut it out.
Step 2 Mark position of cleat on stringer and remove. Cut a new cleat using 90 x 45mm treated pine. Hold against stringer so it matches position of original. Drill pilot holes for batten screws, then screw in place.
Step 3 Cut new treads to length using 140 x 45mm treated pine. Position on cleats, predrill, then screw through into cleats. For housed treads, knock tread into housing from behind, if possible.
STEP 4 Cover step with 2 coats of outdoor paint, allowing to dry between coats.